1. Field of the Invention
This invention relates to textiles in general, and in particular to an electrically-conductive textile fiber for use in the fabrication of antistatic fabrics of various kinds.
2. Prior Art
The accumulation of static electricity as a result of the utilization of fabrics is a phenomenon which has commanded the attention of the textile industry for some time. The presence of static is a cause not only of annoyance (e.g. items of apparel cling to the body and are attracted to other garments; fine particles of lint and dust are attracted to upholstery fabrics, increasing the frequency of required cleaning; one experiences a jolt or shock upon touching a metal doorknob after walking across a carpet), but also of danger (e.g. the discharge of static electricity can result in sparks capable of igniting flammable mixtures such as ether/air, which are commonly found in hospitals, especially in operating rooms). All of these effects are accentuated in atmospheres of low relative humidity.
Of the many proposals for preventing the undesirable buildup of static electricity, the most satisfactory, with respect to their efficiency and permanence, appear to be those which comprehend the utilization of fibers possessing electrical conductivity (e.g. metal fibers, fibers coated with electrically conductive material, or metallic laminate filaments) in combination with common natural and man-made fibers to produce a woven, knitted, netted, tufted, or otherwise fabricated structure, which readily dissipates the static charges as they are generated. Some of the more noteworthy of these methods and structures and some pertinent references in considering the present application may be found in U.S. Pat. Nos. 2,129,594; 2,357,851; 2,714,569; 3,069,746; 3,288,175; 3,582,444; 3,582,445, 3,582,448, 3,586,597; 3,590,570; 3,706,195; 3,708,335; 3,729,449; 3,669,736; 3,726,955; 3,669,726; 3,129,053; 2,734,978; 2,566,441; and 2,473,183; in Webber, "Metal Fibers," Modern Textiles Magazine, May, 1966, pp. 72-75; in Belgian patents 775,935 and 790,254; in French Pat. No. 2,116,106; in British Pat. Nos. 354,233; 1,391,262; 1,417,394; in Japanese Utility Model Sho 42-21017; and Japanese patent 45-29920 filed May 25, 1967, issued Sept. 29, 1970.
Notwithstanding the efficacy of these and similar expedients, they are found lacking in certin important aspects, viz:
The manufacture of metallic fibers of fine denier, especially in the form of monofilaments, is a difficult and costly operation; and since such fibers are quite dissimilar in character from ordinary textile fibers, problems arise in connection with blending and processing, as well as in the hand of the products obtained.
Metallic laminate filaments, on the other hand, do not present blending and processing problems, because of their close similarity to ordinary textile fibers, and the hand of the products obtained is consequently not objectionable. However, the cost of such filaments is high when compared with the natural or man-made fibers with which they are blended.
Textile fiber substrates the surfaces of which have been coated by vapor deposition or electrodeposition, or by the application of adhesive compositions containing finely divided particles of electrically conductive material, are in some cases less costly than metal fibers and/or metallic laminate filaments, depending upon the nature of the electrically conductive material employed. However, such coatings are often found lacking in cohesion and adhesion and are frequently too thick to be practicable in some applications--especially when the nature of the electrically conductive particulate matter is such that a high concentration thereof is required for satisfactory conductivity. Economy is achieved, therefore, only through sacrifices in durability of conductivity of the fiber.
The extrusion of powered synthetic polymer/finely divided electrically conductive material blends into filaments or as extruded coatings on a filamentary substrate having the same or a different polymeric composition is also well known. Unfortunately, blends requiring a high concentration of the electrically conductive material are often not readily extruded, if at all, and any filaments and filamentary coatings which are produced therefrom are totally lacking in cohesion and adhesion.
References such as U.S. Pat. No. 2,357,851 and Belgian Pat. No. 790,254 comprehend either (a) total incorporation of particles in the fiber substrate (as would be obtained by the spinning of a solution or a melt containing them), or (b) surface incorporation of particles, achieved by the use of (1) an adhesive--e.g., to produce a true coating--or (2) impregnation of the particles into a fiber surface which has been softened or made tacky by means of a liquid softening agent and/or heat. The total incorporation of particles in the fiber substrate and the use of an adhesive to bind the particles to the fiber substrate as a true coating have been previously discussed as inadequate and undesirable, because of (1) loss of physical properties as a result of disruption of the structural integrity of the fiber and (2) a low degree of durability, respectively. Moreover, in the present invention, the surface of the fiber is dissolved, not softened. It is well known in the art that the dissolution mechanism is not the same as the mechanism of softening or swelling whether accomplished by heat or by means of softening (swelling) agents. When a polymeric material is softened or swollen by a liquid, the liquid is imbibed into the polymer to produce an expanded gel. It is only when a true solvent is used as the liquid that the expanded gel disintegrates to produce a true solution. That a true dissolution occurs in the present invention is clear from the examples of solvents employed: e.g., for nylon 6 (polycaprolactam)--concentrated formic acid and sulfuric acid. In the present invention, it is only because the solvent is removed before the structural integrity of the filament is destroyed (viz., almost immediately after application thereof) that the process has any utility. That such can be accomplished is considered unobvious. Furthermore, it is only because there is a true dissolution of the surface of the filament that the electrically conductive particles present as a dispersion in the solvent can suffuse uniformly into the surface of the filament, and later, after removal of the solvent, present a uniform dispersion rather than a mechanical penetration of electrically conductive particles, which results in no significant loss in physical properties while providing sufficient and durable conductivity for many applications.
In this regard, Japanese Pat. No. 45-29920 (1970), which discloses the incorporation of abrasive sands into the surface of plastic sheets by the application thereto of a dispersion of the sand in a solvent for the sheet, does not render the present invention obvious, since it is clear that the Japanese inventors had no concern that the structural integrity of their film would be destroyed (there is no recitation concerning the importance of removal of the solvent after application thereof). On the other hand, such removal of solvent is crucial in the process of the present application, as is clear from the instant specification and claims. That is to say, if a dispersion of conductive particles in a solvent for nylon 6 filaments were simply applied to such filaments, as might be suggested by the reference, and no provisions were made for almost instantaneous removal of the solvent, the filament would almost immediately disintegrate completely. To be sure, it is only because the solvent is removed before the structural integrity of the filament is destroyed (viz., almost immediately after application thereof) that the process of the present application has any utility. Moreover, in view of the background of the art, it is not at all obvious that such could, in fact, be accomplished. Indeed, other inventors dealing with fibers and filaments have chosen liquids which soften, rather than dissolve the surface of the fibers or filaments. In this regard, reference is again made to U.S. Pat. No. 2,357,851 and Belgian Pat. No. 790,254.